Re: [PATCH v4] ceph: invalidate pages when doing direct/sync writes
From: Xiubo Li
Date: Thu Apr 07 2022 - 19:52:13 EST
On 4/8/22 4:21 AM, Jeff Layton wrote:
On Fri, 2022-04-08 at 03:24 +0800, Xiubo Li wrote:
On 4/8/22 3:16 AM, Jeff Layton wrote:
On Fri, 2022-04-08 at 03:03 +0800, Xiubo Li wrote:
On 4/7/22 11:15 PM, Luís Henriques wrote:
When doing a direct/sync write, we need to invalidate the page cache in
the range being written to. If we don't do this, the cache will include
invalid data as we just did a write that avoided the page cache.
Signed-off-by: Luís Henriques <lhenriques@xxxxxxx>
---
fs/ceph/file.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)
Changes since v3:
- Dropped initial call to invalidate_inode_pages2_range()
- Added extra comment to document invalidation
Changes since v2:
- Invalidation needs to be done after a write
Changes since v1:
- Replaced truncate_inode_pages_range() by invalidate_inode_pages2_range
- Call fscache_invalidate with FSCACHE_INVAL_DIO_WRITE if we're doing DIO
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 5072570c2203..97f764b2fbdd 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -1606,11 +1606,6 @@ ceph_sync_write(struct kiocb *iocb, struct iov_iter *from, loff_t pos,
return ret;
ceph_fscache_invalidate(inode, false);
- ret = invalidate_inode_pages2_range(inode->i_mapping,
- pos >> PAGE_SHIFT,
- (pos + count - 1) >> PAGE_SHIFT);
- if (ret < 0)
- dout("invalidate_inode_pages2_range returned %d\n", ret);
while ((len = iov_iter_count(from)) > 0) {
size_t left;
@@ -1938,6 +1933,20 @@ ceph_sync_write(struct kiocb *iocb, struct iov_iter *from, loff_t pos,
break;
}
ceph_clear_error_write(ci);
+
+ /*
+ * we need to invalidate the page cache here, otherwise the
+ * cache will include invalid data in direct/sync writes.
+ */
+ ret = invalidate_inode_pages2_range(
IMO we'd better use truncate_inode_pages_range() after write. The above
means it's possibly will write the dirty pagecache back, which will
overwrite and corrupt the disk data just wrote.
I disagree. We call filemap_write_and_wait_range at the start of this,
so any data that was dirty when we called write() will be written back
before the sync write.
If we truncate the range, then we'll potentially lose writes that came
in after write was issued but before truncate_inode_pages_range. I think
we'd rather let what we just wrote be clobbered in this situation than
lose a write altogether.
All of this is somewhat academic though. If you're mixing buffered and
direct writes like this without some sort of locking, then you're just
asking for trouble. The aim here is "sane behavior to the best of our
ability", but we can't expect it to always be sane when people do insane
things. ;)
Just in the case Luis hit. Before writing the new data the mapping
happen when reading the src in copy_from_usr(). So once the writing done
the pagecache is caching the stale contents.
Not just in that case.
You could have 2 unrelated processes, one doing DIO writes and one doing
mmap writes. You're likely to end up with a mess unless you're very
careful with what you're doing, but there should be some expectation
that it will work if you serialize things correctly and/or have them
writing to their own areas of the file, etc.
For this case I checked the other use cases, they are seems will do:
filemap_invalidate_lock(inode->i_mapping);
write pagecache back;
invalidate the mapping and drop the pages;
do the IOs;
filemap_invalidate_unlock(inode->i_mapping);
The filemap_invalidate_lock could prevent the page fault to map them
again during this.
In any case, we'll never get perfect cache coherency, and I figure that
until the write returns, what's in the pagecache ought to be considered
valid.
Okay, I am okay with this.
As my understanding is that we should make sure that the pagecache is
always valid during the sync write, or if the pagecache will be
revalidated it should just block the other processes to read from the mmap.
-- Xiubo
Though it seems impossible that these pagecaches will be marked dirty,
but this call is misleading ?
Not impossible at all. You can open a file O_DIRECT and then mmap the fd
for PROT_WRITE (or just open the file a second time and do it).
We definitely recommend against mixing buffered and direct I/O, but
nothing really prevents someone from doing it. If the user is properly
using file locking, then there's really no reason it shouldn't work.
+ inode->i_mapping,
+ pos >> PAGE_SHIFT,
+ (pos + len - 1) >> PAGE_SHIFT);
+ if (ret < 0) {
+ dout("invalidate_inode_pages2_range returned %d\n",
+ ret);
+ ret = 0;
+ }
pos += len;
written += len;
dout("sync_write written %d\n", written);